This invention relates generally to the in situ and non-destructive examination of large circumferential surfaces, particularly including welds, and more particularly, obstructed and recessed peripheral welds. Such surfaces and welds may be found throughout boiling water nuclear reactors and, in particular, welds between ring structures that support the core plate and the core shroud arranged above the ring structures, sometimes referred to as the H6A weld.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide, sometimes referred to as a grid, is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. The core shroud is a reactor coolant flow partition and structural support for the core components. In most instances, the core shroud will have a generally cylindrical shape and surround both the core plate and the top guide. A removable shroud head is coupled to a shroud head flange at the top of the shroud.
Above the H6A weld, the core plate will typically be spaced from the shroud using a series of irregularly spaced core plate wedges set into a thin annular opening formed between the core plate and the inner surface of the shroud. The core plate wedges obstruct access to the welds and surfaces within the annular opening and the irregular spacing between the core plate wedges further complicates access. During operation of the reactor, however, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the welds. In particular, lateral seismic/dynamic loading could cause relative displacements at cracked weld locations and may produce leakage and misalignment of reactor components that could compromise the safety or performance. Given the complex configuration of the attachment between the shroud and core plate, however, in situ examination of the welds has proven very difficult.
It is desirable, therefore, to provide an apparatus and a corresponding method for inspecting the welds used to attach the shroud and the core plate to support rings arranged below the core plate that is reliable and is capable of examining the majority of the circumference of such welds and the associated surfaces. When using ultrasonic sensors to examine a weld, the focus point, direction and frequency of the ultrasonic beam may be selected to align with a predetermined fusion line between a weld and the attached structures. The ultrasonic beam may then be repeatedly refocused to move the focal point along the weld fusion line in discrete increments of about 0.25 to about 12.7 mm (about 0.01 to about 0.5 inch). One method for such incremental scanning is disclosed in U.S. Pat. No. 6,332,011, the contents of which are hereby incorporated by reference.
A variety of mechanisms have been devised for the examination of welds, particularly for use in hostile environments such as the interior of RPVs. One such apparatus is disclosed in U.S. Pat. No. 5,568,527, the contents of which are hereby incorporated by reference, and provides a remotely operated apparatus with clamping, sliding, rotational and sensor mechanisms to scan an ultrasonic transducer over specific core spray “T-box” welds including the T-box to cover plate attachment the T-box to thermal sleeve attachment welds.